Blank entry when inserting into mysql (database) - php

Question

i am trying to create a login and registration page using php and mysql but i met with some problem.
I reference to this video and the code is below (the code is incomplete, i only did for registration).
So the problem is that when i submit an entry using the register side, my database shows a blank record. I tried various method like
$reg = "INSERT INTO usertable (user,pwd) values ('".$user."','".$pwd."')";
but it did not work and when i did this:
$reg = "INSERT INTO usertable (user,pwd) values ('ABC','1234')";
it worked. What should i do to insert entry using the input text?
<?php
session_start();
$conn = mysqli_connect('localhost', 'root', '1234');
mysqli_select_db($conn,'registeration');
$user = $_POST["user"];
$pwd = $_POST["pwd"];
$s = "select * from usertable where user = '$user'";
$result = mysqli_query($conn,$s);
$num = mysqli_num_rows($result);
if($num == 1){
echo"Username Already Taken";
}
else{
$reg = "INSERT INTO usertable (user,pwd) values ('$user','$pwd')";
mysqli_query($conn,$reg);
}
?>
<h2> Register Here </h2>
<form action="index.html" method="post">
<label>Username</label>
<input type="text" name="user" required>
<label>Password</label>
<input type="text" name="pwd" required>
<br><button type="submit"> Login </button>
</form>

Answer 1

A easy way to know what's full SQL query string the program or you have made.
$reg = "INSERT INTO usertable (user,pwd) values ('$user','$pwd')";
echo $reg
It is better use {} to instead of . to insert variable into a string.
$reg = "INSERT INTO usertable (user,pwd) values ('{$user}','{$pwd}')";
To be sure the POST/GET action target is correct,
For your current code I am not sue index.html can handle it, probably it should be $_SERVER["PHP_SELF"]
To understand PHP String Operators, please refer to
https://www.php.net/manual/zh/language.operators.string.php

Answer 2

I found my mistake. I wrote my php and the register table (together in registration.php) however, my form redirects me to index.html. I copied and pasted my php into my index.php (changed the name) and it worked.
Thanks for those who helped me!
<h2> Register Here </h2>
<form action="index.php" method="post">
<label>Username</label>
<input type="text" name="user" required>
<label>Password</label>
<input type="text" name="pwd" required>
<br><button type="submit"> Login </button>
</form>